1. Field of the Invention
The present invention relates to a semiconductor device having an improved voltage controlled oscillator circuit formed by connecting a variable-capacitance element in combination with a bipolar transistor on a same single semiconductor substrate. More specifically, a variable-capacitance element is formed using a P layer as the base and an N layer as the collector among the NPN layers for forming a bipolar transistor.
2. Background Art
FIG. 3A shows an example of a conventional voltage control oscillator circuit formed by combining a bipolar transistor and a variable-capacity element formed on a  same semiconductor substrate. FIG. 3B is a plan view showing the constitution of the variable-capacity element surrounded by broken lines in FIG. 3A; FIG. 3C is a sectional view showing the side structure along B-B line in FIG. 3B; and FIG. 3D is a small signal equivalent circuit of the variable-capacity element.
As these drawings show, when a bipolar transistor 10 and a variable-capacity element 30 are composed on the same semiconductor substrate 6, as FIG. 3C shows, an N+ layer 7 is formed as a contact on the semi-insulating semiconductor substrate 6. An N layer 8 and a P layer 9 are formed thereon to be a collector and a base of an NPN transistor. A wiring 1 and a connecting wiring 3 are formed on the base layer 9, and the electrodes 2 of the collector layer 8 are formed on the contact layer 8, and wirings 5 for connecting the electrodes 2 to other circuit elements are formed on the both sides of the collector layer 8. As FIG. 3B shows, the wirings 5 connect the both electrodes 2 mutually.
Furthermore, circuit elements shown in FIG. 3A are provided on the semiconductor substrate 6 to form the voltage control oscillator circuit. Specifically, a capacitor 12 for isolating direct-current components from external circuits is connected to the base of an NPN bipolar transistor 10 that forms the active part of the oscillator circuit, and each inductor 11a and 11b that determines the feedback quantity to the bipolar transistor is connected to the capacitor 12 and the emitter of the bipolar transistor 10.
The wiring 5 connected to the collector layer 8 of a variable-capacity element 30 is connected to the inductor 11a on the base side of the bipolar transistor. The serially connected body of the control power source 13 and the choke coil 14 for isolating the alternate-current components from a control power source 13 are connected also to the inductor 11a. The reference numeral 15 denotes the output terminal of the oscillator circuit.
In such a constitution, the variable-capacity element 30 utilizes the phenomenon wherein the capacity generated between the base layer 9 and the collector layer 8 that form the PN junction varies corresponding to the variation of the voltage of the control power source 13 (e.g., see Japanese Patent Application Laid-Open No. 2000-124473).
The conventional semiconductor device is constituted as described above, and utilizes the variation of capacity generated between the base layer 9 and the collector layer 8 of the PN-junction diode as the variable-capacity element 30. In this case, however, when a forward-direction voltage is applied to the above-described PN-junction diode, the depletion layer generated from the base layer 9 toward the direction of the contact layer 7 shrinks. Therefore, the collector region, which is from the lower end of the depletion layer to the contact layer 7, expands in FIG. 3C. Accordingly, the resistance component 18, shown in FIG. 3D, which is serially connected to the resistor 16 and the capacity component of the capacitor 17 of the diode, increases, and the Q value of the resonant circuit was lowered. Thus, the noise characteristics of the voltage-controlled oscillation circuit is deteriorated.
The object of the present invention is to solve the above-described problem, and to provide a semiconductor device that can form a voltage control oscillator circuit having a variable capacity element 30, using PN junction, of a small resistance component serially connected to a capacity component on the same semiconductor substrate 6 with a bipolar transistor 10.